A tractor/baler combination is used in agriculture to take up crop lying in a swath on a field with a pick-up and to press and bind the crop into a bale in a baling chamber. Due to different widths of the baling chamber and the swath, it is appropriate to steer the tractor in a manner such that the pick-up of the baler is not continuously centered over the swath, but moves over time successively to the left and to the right side of the swath, in order to produce a cylindrical bale. Thus, sensors for the shape or deformability of the bale have been proposed that provide steering information to steered wheels of the baler (U.S. Pat. No. 4,702,066 A) or pivoting the tongue of the baler (U.S. Pat. No. 4,433,533 A) to steer the baler laterally with respect to the tractor or giving a steering indication to the tractor operator (FR 2 579 063 A, U.S. Pat. No. 4,224,867 A, U.S. Pat. No. 4,855,924 A, EP 1 593 299 A) in a manner to drive on a sinusoidal path along the swath for obtaining a homogenous bale shape. It was also proposed to steer the tractor based upon a bale shape signal and a detection of a swath position (EP 1 634 491 A, EP 1 685 759 A, EP 1 813 146 A).
Mechanical arrangements for detecting the size or the symmetry of a bale produced in a rotary baler have been proposed in EP 1 685 759 A, U.S. Pat. No. 4,433,533 A and FR 2 579 063 A. These references disclose two feelers or rolls that interact with belts enclosing the baling chamber.
EP 0 634 094 A teaches that the compactness of the bale can be detected using a spring in contact with the side flanks of the bale.
EP 1 634 491 A describes a baler with three ultrasonic distance sensors distributed over the width of the baling chamber for detecting and displaying the bale diameter in three different positions.
US 1 819 H describes a baler shape monitor with three rollers sensing the bale diameter distributed over the width of the baling chamber. A switch is actuated and thus a signal is provided to the operator once the diameter of the bale in one of the end locations is less than the diameter at the intermediate location.
U.S. Pat. No. 4,850,271 A describes a rotary baler capable of measuring the diameter of the bale and displaying this information. The sensor assembly includes three sensors positioned along the width of the baling chamber, each of which detects the tension in the belts of the baling chamber by use of a spring-loaded mechanism actuating a potentiometer. The mechanism moves potentiometers that are connected to the display arrangement by means of a signal processor. An additional potentiometer detects the position of a tensioning arm of the belts. The information regarding the bale shape is thus derived from an indirect measurement of the belt tension.
A disadvantage of the above-described electro-mechanical sensors for sensing the bale diameter, comprising mechanical feelers coupled to potentiometers, is that the mechanical connection between the feeler and the potentiometer is subject to tolerances. In order to achieve a correct indication of the correct bale size and shape, a calibration or adjustment of the electro-mechanical sensors is thus necessary. This calibration is, in the prior art, performed mechanically during the assembly process of the baler or during service work, and comprises a calibration of an offset (indicating an empty baling chamber) and a slope (corresponding to a known bale size) for at least two sensors. The calibration is cumbersome and thus one can frequently find balers running with an improper calibration of the bale size sensors. A calibration is also useful when contactless sensors are used, like in EP 1 634 491 A, since the mounting location of the sensor in the baler can vary from baler to baler.